Ferritic-nitrocaburizing is becoming a more popular process to improve a part’s mechanical properties and corrosion resistance on plain carbon and low alloy constructional steels. Advertised as a major contribution to enhanced corrosion resistance, post-oxidation of ferritic-nitrocarburized steel is used to prevent/delay corrosion in service. There are different methods of oxidizing the steel after this heat treatment process. Different trademarked processes claim to provide more corrosion resistance than other similar processes. This paper focuses on the post-oxidation process specifically and compares several of the most common oxygen-containing media used for post-oxidation to determine if corrosion resistance is a function of the oxygen-bearing media or a function of the oxygen percentage during the process. In addition to comparing oxidized samples, rust preventative oil (RPO) is compared as most commercial Ferritic-nitrocarburizing with Post-Oxidation processes involve the application of an RPO after the process as well. Salt-spray testing determined that none of the oxygen-bearing media are significantly better than another when all other variables are held relatively constant. It also shows that parts coated with RPO perform significantly better in salt-spray testing than the same “dry” parts without any additional protective coatings. The salt-spray results also indicated that there is no significant difference in corrosion resistance between parts with and without the post-oxidation process.

Studying the gas composition of an atmosphere furnace reveals many parameters that can be used to predict metallurgical results of parts being processed. Today’s heat treaters strive for process consistency in order to eliminate variability, rework, and (in the worst case) scrap. In atmosphere heat treating, understanding control parameters and atmosphere composition provides critical insight into expected results. In this presentation, the audience will have the opportunity to observe gas compositions that would typically be found in carbon-neutral or carbon-rich atmospheres; they will also be able to see how atmospheric properties directly affect the metallurgical properties of heat treated parts. The presenter will illustrate examples of common atmospheres based on furnace types and will discuss common characteristics of improperly controlled atmospheres and how those atmospheres can lead to flawed results. Real-world examples of metallurgical results will be used to illustrate what can be expected from an “out of control” situation.